Lead-acid batteries have evolved, over time, as the demands for a source of mobile electric power have grown. In certain service applications, the flooded lead-acid battery may be operated in a partial state of charge (PSoC), for example, an approximately 50 to 80% state of charge, which is unlike the typical SLI (starting, lighting, and ignition) battery which is usually operated at 100% state of charge. For example, the hybrid electric vehicle's (HEV's) battery may operate in a PSoC, for example, at approximately 50 to 80% charge. As such, the battery may undergo shallow charge/recharge cycles, and may not undergo overcharge where dissociation of water evolves hydrogen and oxygen that mixes stratified acid within the cell.
The lead-acid battery is an excellent storage medium for energy, but one of the limitations may be the ability of the battery or battery chemistry to accept charge rapidly, particularly when the battery is at a high state of charge. In PSoC applications, this rapid charge may come from the use of regenerative braking, which may recover much of the energy used in slowing the vehicle. For this reason, the battery may typically be operated at a lower state of charge. In other applications, the battery may operate under highly demanding service conditions; in these instances incomplete charging is routine and may be difficult to avoid. For example, in developing regions of the world, the use of power inverter battery systems is common due to power grid instability. In these instances, battery charging from a discharged state may be solely dependent on unpredictable power grid availability.
Unlike many energy storage chemistries, in lead-acid batteries, the electrolyte, as well as the active materials within the electrode plates (for example, PbO, et al.), take part in the electrochemical reaction. During the electrochemical process lead sulfate is attracted to the negative electrode and is precipitated in the form of seed-crystals. Under typical fully charged operation the crystals remain small and well dispersed on the plate surface. Porosity of the electrode is marginally changed. In PSoC operation, however, the formation of the sulfate crystal is significantly less controlled. The result may be extensive sulfate crystal growth to the extent that electrode porosity is compromised. At this stage, the plate is termed “sulfated” as the crystal formation is irreversible. The ability of the negative electrode to accept charge may be dramatically reduced and eventually end of life of the battery may be reached.
The battery separator is a component that divides, or “separates”, the positive electrode from the negative electrode within a lead-acid battery cell. A battery separator may have two primary functions. First, a battery separator should keep the positive electrode physically apart from the negative electrode in order to prevent any electronic current passing between the two electrodes. Second, a battery separator should permit an ionic current between the positive and negative electrodes with the least possible resistance. A battery separator can be made out of many different materials, but these two opposing functions have been met well by a battery separator being made of a porous nonconductor.
Accordingly, there is a need for new battery separator and/or battery technology to meet and overcome the new challenges arising from current lead-acid battery needs.